Glock pins are numbered: 1, 2, and 3. Starting from the highest to the lowest. All Glock pins remove from the left-hand side of the frame, and reinstall from the right-hand side. (When you're looking down on the frame from the top.) Always remove and reinstall the #1 pin FIRST.

While removing the #2 pin you might have to gently wiggle the slide stop up and down. This is normal assembly/disassembly procedure. Your problem is PROBABLY caused by either a damaged or out-of-place slide stop spring. Here's how a properly located slide stop spring should look:

The slide stop spring should always be UNDERNEATH the #1 pin. If yours is, then that spring has to be either out of its groove, or damaged; and the slide stop, itself, needs to be replaced. Here, this should help you out: